@david According below sources form should be computed on yesterday ctl & yesterday atl:
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Part 3: Training Stress Balance—So What? - Joe Friel
=> “the resulting TSB value is for tomorrow—not for today.”
(This is TSB details from the Joe Friel article you are referencing on the fitness feature itself)
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What is the Performance Management Chart? | TrainingPeaks
=> " by subtracting yesterday’s Fatigue from yesterday’s Fitness we come up with the yellow line, or the Form"
I’m the guy who run Elevate (for strava & desktop app), I also implemented it as is.
Having it as an option could be great.
By the way, amazing work on intervals.icu 
!